linux-kernel - Re: [PATCH v3 2/2] tmpfs: Make tmpfs scalable with percpu

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Message-Id: <20100617210859.40e024a1.akpm@linux-foundation.org>
Date:	Thu, 17 Jun 2010 21:08:59 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	Tim Chen <tim.c.chen@...ux.intel.com>,
	linux-kernel@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>,
	Hugh Dickins <hughd@...gle.com>, yanmin.zhang@...el.com
Subject: Re: [PATCH v3 2/2] tmpfs: Make tmpfs scalable with percpu_counter
 for  used blocks

On Fri, 18 Jun 2010 10:35:51 +0900 Minchan Kim <minchan.kim@...il.com> wrote:

> > __struct shmem_sb_info {
> > __ __ __ __unsigned long max_blocks; __ /* How many blocks are allowed */
> > - __ __ __ unsigned long free_blocks; __/* How many are left for allocation */
> > + __ __ __ struct percpu_counter used_blocks; __/* How many are allocated */
> 
> Just a nitpick.
> Why do you change free_blocks and used_blocks?
> I think we can use free_blocks following as.
> 
> ex)
> if (percpu_counter_compare(&sbinfo->free_blocks, 0))

See previous lengthy discussion ;)

If we count free_blocks then we need to alter the value of free_blocks
in remount_fs, and reinitialising distributed counters on-the-fly is
ugly.

I suppose we could have done it by doing a large add or sub in
remount_fs, and keeping track of the exact value of free_blocks
elsewhere in the superblock, but it's far simpler this way.

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